As is so frequently the case, "experiments of nature," in which patients lack a normal bodily constituent, educate us to the function of the missing component. Although the plasma complement (C) system was initially recognized by its ability to foster hemolysis of antibody-coated RBCs, the microbicidal activity of the system probably has determined its evolutionary selection. Thus, rare patients who are genetically deficient in C3, a C component that is common to both pathways of C activation—the classical and alternative, are chronically and severely victimized by pyogenic infections.1
BENEFICIAL EFFECTS OF COMPLEMENT-GRANULOCYTE INTERACTION
The C system probably serves mainly to potentiate the capability of granulocytes to kill bacteria, and does so in several different ways. Perhaps most noteworthy is the ability of C3b, a cleavage product that results from activation of C3, to opsonize bacteria and other unwanted particles. Since granulocytes (and monocytes) have receptors for this and other
Jacob HS. Granulocyte-Complement Interaction: A Beneficial Antimicrobial Mechanism That Can Cause Disease. Arch Intern Med. 1978;138(3):461–463. doi:10.1001/archinte.1978.03630270075026
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